<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .green{
            color: green;
            font-size: 20px;
        }
        .li{
            text-decoration: underline;
        }
        /* .green,.li{
            background-color: aqua;
        }
        .green.li{
            border: 1px solid #369;
        }
        .green>.li{
            color: red;
        }
        .green .li{
            color: yellow;
        } */
    </style>
</head>
<body>
    <ul>
        <li class="green li">1</li>
        <li class="green">2</li>
        <li class="li">3</li>
        <li class="green"><span class="li">4</span></li>
        <li>5</li>
        <li>6</li>
        <li>7</li>
        <li>8</li>
        <li>9</li>
        <li>10</li>
    </ul>
</body>
</html>
<script>
    var lis=document.querySelectorAll("li")
    for(var i=0;i<lis.length;i++){
        lis[i].onclick=function(){
            // this.style.color='red'
            // this.className='green'
            // this.classList.add('green');
            this.classList.remove('green');
        }
    }
</script>